Studies have continued on the unique 18.4K, pI 5.9 phosphoprotein, prosolin (formerly called pp17), a major cytosolic protein which undergoes rapid phosphorylation in HL60 promyelocytic leukemia cells when their growth is inhibited and they are induced to differentiate in response to treatment with phorbol ester (TPA). The expression and phosphorylation of prosolin in human peripheral lymphocytes (PBL) was investigated. Prosolin was identified in PBL, and its expression was found to be correlated with the S-phase of the cell cycle. In proliferating PBL prosolin was a major cytosolic component, comprising 0.5% of total cytosolic protein, of which 25% was found in 2 phosphorylated forms. TPA treatment of proliferating PBL caused phosphorylation of about 2/3 of pre- existing unphosphorylated prosolin within 1 hr followed immediately by an abrupt cessation of DNA synthesis. Phosphorylation of prosolin may be an initiating event in the antiproliferative response to TPA. The availability of prosolin only during S-phase may provide a regulatory mechanism by which naturally occuring cytokines may terminate lymphocyte growth, through activiation of antiproliferative processes linked to phosphorylation of prosolin without inhibiting activation of Go or G1 cells.